Climatic origin and plasticity shape emergence timing and fitness in bees and wasps under experimental climate regimes

Abstract Climate warming threatens the persistence of insect populations by forcing them to adjust their phenology—responses that may be constrained by local adaptation and involve physiological trade‐offs. We investigated how hymenopterans with contrasting seasonal phenology and from different climates adjust their emergence timing under current and projected future climates. We also examined the consequences of these adjustments on body mass at emergence as a key fitness trait. We analysed the emergence timing of 14,921 individuals from five cavity‐nesting bee and wasp species exposed to cold, warm and hot post‐winter temperature treatments in a common garden experiment to identify potential genotype–environment interactions. Insects had developed at 161 sites of origin across southern Germany covering multi‐annual mean temperatures (MAT) of 5.9 to 10°C and differing in the temperature deviation (ΔT) that insects experienced during development in the pre‐emergence year relative to MAT. Emergence timing was highly plastic to post‐winter temperatures, with insects emerging earlier in warmer treatments. However, emergence was modulated by MAT and ΔT, suggesting genetic adaptation to long‐term climatic conditions and adjustments to short‐term temperature deviations. For spring‐emerging species, individuals from sites with higher MAT and warmer treatments emerged the earliest (cogradient variation). In contrast, the latest summer‐emerging species exhibited countergradient variation: in the cold treatment, individuals from higher MAT emerged later than those from lower MAT. In spring species, mass loss was higher in warmer post‐winter treatments, with the strongest reductions observed in cool‐adapted individuals. Mass loss was particularly rapid for summer females in warmer treatments, with individuals emerging later losing up to 34% of their mass. However, body mass of summer insects was independent of MAT and ΔT. Our results demonstrate high plasticity of cavity‐nesting Hymenoptera to post‐winter temperatures but also suggest that local adaptation and responses to early‐life temperature can compromise fitness under rapid climate changes. This large‐scale experimental study highlights the complex drivers of insect emergence phenology and fitness and suggests that cool‐adapted, spring‐emerging species may be most vulnerable to ongoing climate warming. Read the free Plain Language Summary for this article on the Journal blog.